Proving the Non-one-to-one Property of f(x)=x^3-3x^2+2x on (-k,k)

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In summary, to prove that f(x)=x^3-3x^2+2x is not one to one on (-infinity,+infinity), we can find the largest value of k such that f(x) is one to one on the interval (-k,k) and use the quadratic formula to solve for the intervals where the derivative of f(x) changes sign, proving that the function is not one-to-one.
  • #1
farmd684
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Homework Statement


show f(x)=x^3-3x^2+2x is not one to one on (-infinity,+infinity)


Homework Equations



finding the largest value of k such as f is one to one on interval (-k,k)

The Attempt at a Solution


i can get f`(x)=3x^2-6x+2 but it is positive so f(x) should be one-to-one
how to prove it
 
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  • #2
f(x)=3x^2-6x+2 isn't positive. f(1)=-1.
 
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  • #3
I just automatically looked at the discrimant for 3x2- 6x+ 2:
[tex]\sqrt{6^2- 4(3)(2)}= \sqrt{36- 24}= \sqrt{12}[/tex]
Since that is a real number, the 3x2- 6x+2 changes sign, and the original function changes "direction", at two places.
 
  • #4
HallsofIvy said:
I just automatically looked at the discrimant for 3x2- 6x+ 2:
[tex]\sqrt{6^2- 4(3)(2)}= \sqrt{36- 24}= \sqrt{12}[/tex]
Since that is a real number, the 3x2- 6x+2 changes sign, and the original function changes "direction", at two places.

thanks but how can i do this
largest value of k such as f is one to one on interval (-k,k)
 
  • #5
A function f(x) is one-to-one as long as its derivative does not change sign- and a continuous derivative, such as the derivative of any polynomial, can change sign only where the derivative is 0.

Solve 3x2- 6x+ 2= 0, say by using the quadratic formula. Those 2 values will give 3 intervals on which the function is one to one. One of them contains the an interval of the form (-k, k).
 

1. How do you prove the non-one-to-one property of a function?

To prove the non-one-to-one property of a function, you need to show that there exist two distinct inputs that result in the same output. In other words, the function is not injective and therefore not one-to-one.

2. What is the function f(x)=x^3-3x^2+2x?

The function f(x)=x^3-3x^2+2x is a polynomial function with a degree of 3. It is a cubic function that can be graphed as a curve.

3. What does the interval (-k,k) mean in this function?

The interval (-k,k) represents a range of values for the input variable x. In this function, it means that the function is defined for all values of x between -k and k, including -k and k.

4. How do you prove that a function is not one-to-one?

To prove that a function is not one-to-one, you can use a counterexample. This means finding two distinct inputs that result in the same output. In this case, we can find two values in the interval (-k,k) that give the same output for f(x)=x^3-3x^2+2x.

5. What is the significance of proving the non-one-to-one property of a function?

Proving the non-one-to-one property of a function is important because it tells us that the function is not invertible. This means that there is no unique inverse function for this function, and we cannot solve for x in terms of y. It also helps us understand the behavior of the function and its relationship between inputs and outputs.

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